U.S. automakers disagree sharply over how many cars Japan should be allowed to import into the United States during a fourth year of quotas, industry executives and trade officials said yesterday.

General Motors Corp. wants an increase of at least 300,000 Japanese cars sent to the United States under voluntary export quotas, while Ford Motor Co. favors leaving the quota at 1.68 million cars, the limit since 1981, and Chrysler has told U.S. trade officials to press for a longer extension of the quotas than the Japanese now seem prepared to accept.

GM officials said that increasing the import ceiling to 2 million autos is needed to help GM carry out its plans for bringing small Isuzu and Suzuki passenger cars into the U.S. market in the summer and fall of 1984. About 90,000 Suzukis and 200,000 Isuzus would be sold as 1985 models under GM's Chevrolet and Pontiac labels, according to company officials.

The voluntary restrictions were instituted in 1981 to quell growing cries in the United States for mandatory quotas on imports, which consumed 27.8 percent of the domestic auto market. Japanese cars accounted for slightly more than 22 percent of the foreign share.

Now U.S. trade officials are saying they expect Japan to agree to quotas again in 1984. "There is no question" that the Japanese will accept a fourth year of restraints, one official said last week. "All the signals point that way."

GM President F. James McDonald agrees with that assessment. Continuing high U.S. unemployment (9.5 percent), upcoming national elections, and increasing agitation in Congress for stronger trade barriers probably will influence the Japanese to go along with a fourth year of restraints, McDonald said yesterday in Detroit.

Japan wants to raise the export ceiling to 2.5 million cars annually if restraints are continued. But the Japanese want to exclude made-for-GM imports from that number.

GM's efforts to bring in more Japanese cars does not sit well with its two main domestic competitors, Ford Motor Co. and Chrysler Corp.

The current quota level "didn't really do what was anticipated" in holding down the Japanese share of the U.S. auto market, said David M. McCammon, Ford's vice president of corporate strategy and analysis. "We think the quota level should stay where it is," he said.

Meanwhile, Harley-Davidson, America's only maker of motorcycles, acknowledged reports from Tokyo yesterday that it had been negotiating for long-term loans from Japanese motorcycle manufacturers to allow it to develop a new line of smaller motorcycles competitive with Japan's most popular exports. These loans would replace stiff 49.4 percent tariffs the U.S. slapped on big motorcycles to allow Harley Davidson to get out from under Japanese competition.

But a Harley-Davidson spokesman said the Japanese had initiated the talks, which "have not produced results to date." According to Nihon Keizai Shimbun report from Tokyo, the loans would amount to about $33 million.